GB2299654A

GB2299654A - Cooling system

Info

Publication number: GB2299654A
Application number: GB9506835A
Authority: GB
Inventors: Wei-Min Zhang
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-04-03
Filing date: 1995-04-03
Publication date: 1996-10-09
Anticipated expiration: 2015-04-03
Also published as: GB9506835D0; GB2299654B

Abstract

Disclosed is a cascade cooling system 10 for refrigeration or air-conditioning, comprising a vapour-compression cooling circuit 100; a brine circuit 200 thermally coupled to an evaporator 150 of the cooling circuit; and a coolant circuit (300) having a column (310) accommodating and thermally engaging a compressor assembly 110 and a condenser 120 of the cooling circuit. The coolant circuit 300 is also connected to a heat storage tank 400 for absorbing heat by the coolant and temporarily storing the same as latent heat in the tank 400. The coolant has aqueous ammonia and endothermic salt components, such as ammonium nitride or urea, to provide excellent cooling by the joint effects of evaporation and endothermic dissolution. The column 310 has baffle means (116, 315, not shown) for stabilising an axial temperature gradient, and flow guide means (312) for circulating endothermic salt. The antifreeze in the circuit 200 is also used as a cold storage material when the system 10 is switched by a unit 500 to a cold storage mode. Thermoelectric means (112, 610) are used at key positions in both the coolant circuit 300 and the antifreeze circuit 200 to improve operation flexibility and efficiency. Also disclosed is a method for selectively control the system operation according to whether a low-cost electricity is available, to further maximise the efficiency.

Description

Cooling System Technical Field of Invention The present invention relates to a cooling system suitable for refrigeration or airconditioning, and more particularly, to a system having a cooling mechanism combined with a coolant circulating circuit to achieve a significantly improved energy efficiency.

Background of Invention It is known that conventional vapour-compression type cooling systems, which have been widely used for both domestic and commercial utilities, are responsible for serious damages to the environment. Generally speaking, the damages are in the following three main aspects. Firstly, these systems, from a global viewpoint, consume a huge quantity of power because they need to be kept in operation 24 hours a day for all the year round (in case of refrigerators or freezers) or for the whole season (in case of air-conditioners). This huge consumption contributes indirectly to the accumulation of greenhouse gases produced during power generation. Secondly, these systems use CFCs or HCFCs as refrigerant which cause direct damages to the ozone layer.Efforts have been made to replace them by new refrigerants, however, even if the replacements can prove to be less hazardous to the ozone layer, as unnatural substances they may cause other unknown environmental problems.

Thirdly, a conventional system uses a considerable amount of metal materials such as copper or aluminium for making components of good heat exchange capability, ie. condenser and evaporator, which consume natural resources by themselves, therefore have their shares in causing environment damages and global warming.

There is no need to mention that the damages to the nature caused by human activities are already extremely serious. Something has to be done urgently and effectively for the sake of the long term survival of the whole ecological system, including the human beings and all the other living creatures on this planet.

Summary of Invention The present invention is made as an effort to alleviate, or at least to slow down the general trend of the devastating damages being made to the fragile ecological system by human activities under the name of "economic developments".

It is, therefore, a main object of the present invention to provide a cooling system, and components for it, which are environment-friendly, ie. easy to manufacture, use less environment hazardous materials, and operate with high energy efficiency.

According to one aspect of the present invention, there is provided a cooling system comprising: a cooling mechanism for transferring heat from a cold-generating member to a heat-rejecting member, and a coolant circuit thermally engaged with said heat-rejecting member; wherein a coolant solution with at least one evaporable component is used to circulate in said coolant circuit which has means for promoting evaporation of said evaporable component of said solution.

Preferably, the coolant solution further comprises at least one endothermic salt for producing cooling effects to said heat-rejecting member by endothermic dissolution. The system can have a brine circuit for dissipating cold energy. Preferably, said brine is also used as a phase-change cold storage material when the system is supplied with a low cost electricity. The system can further comprise a heat storage tank thermally engaged with the coolant circuit to work as a buffer for receiving heat from the vapour of said evaporable component. It is desirable that environmentally benign refrigerant, such as carbon dioxide, ammonia or propane, can be used in the system to avoid environment damages.

According to another aspect of the invention, there is provided a gas compression assembly for a cooling system, said assembly comprising: a plurality of compressors serially connected to form a multistage gas compression passage for a refrigerant to be compressed progressively therein, and a coolant passage for circulating a coolant fluid; wherein said gas passage and coolant passage are arranged alongside and being thermally coupled to each other so that the heat generated in the gas passage by compressing the refrigerant can be absorbed and carried away by said fluid circulating through said coolant passage.

According to yet another aspect of the invention, there is provided a circuit for cooling a heat source by circulating a coolant solution of at least two components, said circuit comprising: a heat absorbing portion adapted in thermal coupling with said heat source, and a heat dissipating portion in fluid communication with said heat absorbing portion for dissipating heat absorbed from said heat source; wherein said heat absoibing portion is arranged to have an upper part with a coolant outlet, an lower part with a coolant inlet, and an intermediate zone engaging said heat-generating member, said intermediate zone has baffling means for stabilising a temperature gradient in the coolant flow from said lower end to said upper end so that at least one component of the coolant solution can evaporate and be circulated to said heat dissipating portion via said outlet.

Preferably a thermoelectric member is fitted in the intermediate zone to provide an elevated temperature to promote said evaporation. Also preferably a relatively low pressure is formed in said heat dissipating portion and/or a vapour compressor is connected between said heat absorbing portion and said heat dissipating portion to promote said evaporation.

According to yet another aspect of the invention, there is provided a heat storage tank for a cooling system comprising: a thermally conductive casing containing at least one heat storage material and a fluid passage defined by a thermally conductive and flexible wall member embedded in said heat storage materiaL According to yet another aspect of the invention, there is provided a defrosting arrangement for a cooling system, comprising a thermoelectric member with one thermal pole coupled to a cold-generating member of said cooling system and the other thermal pole coupled to a heat exchange member, and a control unit for selectively changing electric supply to said thermoelectric member to reverse the direction of heat transfer between said cold-generating member and heat exchange member, thereby selectively changing the operation of said arrangement from a frost accumulating mode to a defrosting mode.

It is advantageous that the thermoelectric members are used at key positions of the system to broaden its operation range, to provide flexibility and, at the same time, ensure optimum working conditions for the primary cooling mechanism in the system.

According to yet another aspect of the invention, there is provided a method of operating a cooling system having a brine circulating circuit, comprising steps of.

(a) setting a mode control unit for selecting one of two operational modes in response to the availability of low-cost electricity; (b) operating the cooling system in a first mode when said low-cost electricity is not available, in which the brine is cooled to a first temperature range and circulated; and (c) operating the cooling system in a second mode when said low-cost electricity is available, in which the brine is cooled to a second temperature range low enough for it to be frozen so as to store latent cold energy therein.

It is advantageous that the system of the present invention can be built by using low cost materials, so that in addition to an improved efficiency, the cost is also minimised. It is to be understood that the terms 'low cost materials" or 'low cost electricity" are used in this application in the sense of both low commercial and ecological costs, ie. causing less environment damages and being easily recyclable or being renewable.

Brief Description of Drawings Further features, advantages and details of the present invention are to be described hereinbelow with reference to the preferred exemplary embodiments illustrated in the accompanying drawings, in which: Fig. 1 is a block diagram illustrating basic concepts of the present invention, and the operational and functional relationship between the different components ofthe system; Figs. 2A to 2C are cross-sectional views of a freezer incorporating the principles of the cooling system as shown in Fig. 1; Figs. 3A to 3C are different views showing details and different arrangements of the heat storage tank 400 shown in Figs 1 and 2A; Figs. 4A to 4C are sectional views of different embodiments of the coolant column 310 shown in Fig. 1; Figs. 5A and 5B are sectional views of a defrosting arrangement; and Fig. 6 is a flow-chart illustrating a control method ofthe present invention.

Detailed Description of Preferred Embodiments In Fig. 1, a cooling system 10 according to the present invention has four functional units, represented respectively by blocks 100, 200, 300 and 400, each for a different working medium, but being thermally coupled to form a cascade heat transfer chaim The system 10 also includes a control unit 500 which is electrically connected to these blocks.

The dash-line block 100 is a primary cooling mechanism in the form of a vapourcompression circuit operating under well-known principles. It has a compressor assembly 110, a condenser 120, both physically adapted within a coolant column 310, a dryer 130, an expansion valve 140, an evaporator 150 positioned in a cold storage tank 210, and an accumulator 160. Except the compressor assembly 110 and the condenser 120, the remaining parts ofthe circuit 100 can use commercially available components, so they do not need further description.

The dash-line block 200 represents a brine circulating circuit located within a thermally insulated and closed space. The circuit includes a cold storage tank 210 holding an antifreeze liquid which submerges the evaporator 150 of the above circuit 100. A flow passage is formed by an outlet connection 220, a circulating pump 230 and a return connection 240 which is connected back to the cold storage tank 210.

The dash-line block 300 is a secondary cooling mechanism in the form of a coolant circulating circuit which includes a coolant column 310, a compressor 320 connected between a coolant outlet of the column 310 and an inlet end of a coolant passage 330 arranged in the block 400 which is a heat storage tank. The outlet end of the passage 330 is connected via tubing 340 and a control valve 350 to a coolant inlet ofthe column 310.

A control unit 500 is connected via, respectively, a control line 501 to the compressor assembly 110, a control line 502 to the antifreeze circulating pump 230, a control line 503 to the compressor 320, a control line 504 to the control valve 350 and signal lines 505, 506, 507 and 508 to sensors 511, 512, 513 and 514, each fitted in the heat storage tank 400, closed space 200, cold storage tank 210 and coolant column 310.

Generally speaking, as shown in Fig. 1, heat is transferred in the cascade system 10 from its right-hand side, ie. the block 200, to its left-hand side, ie. the block 400, where it is dissipated to the ambient air. The basic concept of the present invention is to improve the heat exchange of the primary cooling mechanism 100 at both the heat-receiving side, ie. the evaporator 150 where the heat is absorbed, and the heat-rejecting side, ie. the assembly 110 and the condenser 120 where the heat is rejected.When in operation, the evaporator 150 transfers cold energy (ie. negative heat energy) directly to the antifreeze liquid in the cold storage tank 210, and the liquid in turn dissipates, under a forced convection driven by the pump 230, cold energy to contents in the space 200, e.g. when it is a freezer, or to an air flow when it is an air-conditioner. Since a basically water-based liquid is used, which has better thermal conductivity and higher specific heat than air, the arrangement works better than using the evaporator to cool air directly by conduction and natural convection. On the other hand, the heat generated by the compressor assembly 110 is directly transferred to a coolant solution in the coolant column 310, which again provides a more efficient heat transfer than in the case where the heat is transferred directly to the ambient air.The heat exchange efficiency is fUrther improved because the heat rejected by the circuit 100 is used to circulate in the circuit 300 the coolant solution having evaporable components and/or endothermic salt components, so that the coolant circuit 300 can take heat away from the compressor assembly 110 in the form of latent heat of vaporisation which is more efficient than do it in the form of sensible heat, and also cause endothermic dissolution of the salt to cool the refrigerant liquid in the condenser 120.

Since these beneficial effects are produced by using the heat which is conventionally rejected to the ambient as waste, the overall energy consumption is reduced. Any excessive heat is to be temporarily stored in the heat storage tank 400 which serves as a heat buffer to ensure that no part of the system will overheat during its normal operation. By having this heat buffer, the system as a whole can dissipate heat continuously although the compressor assembly operates intermittently. Because of a much improved heat exchange efficiency for both the cold hot sides, physical size of the primary cooling mechanism 100 can be reduced significantly for it no longer needs large heat exchange surfaces formed by long tubing and this leads to a much reduced length of the circulating route actually travelled by refrigerant during a compression cycle.It means the overall flow resistance is reduced, hence a further improvement of efficiency and reduction of cost. Finally, since the size of the circuit is minimi.ced, so is the amount of the refrigerant needed in the circuit, making it easier to meet safety requirements when an environmentally acceptable but toxic and/or flammable refrigerant, such as ammonia or propane, is used in the system It should be noted, that the above explanation regarding the basic concept of the invention is made with reference to a primary cooling mechanism of vapour-compression type. However, the same principles would also apply if a different cooling mechanism is used, e.g. an absorption type, a thermoelectric one (by Peltier effect) or a magnetic one (by thermomagnetic effect).The compression type is preferable because it is by far the most energy efficient and also the most commonly used in existing facilities.

In the following description, details of the block 200 are explained with reference to Figs. 2A to 2C, the heat storage tank 400 with reference to Figs. 3A to 3C, the coolant column 310 with reference to Figs 4A to 4C, a defrosting arrangement 600 with reference to Figs. SA and SB, and a control method is illustrated in the flow-chart of Fig. 6.

Fig. 2A is a cross-sectional view taking along the plane A-A shown in Fig. 2B, while Fig. 2B is a cross-sectional view taking along the plane B-B in Fig. 2A, and Fig. 2C is a cross-sectional view taking along the plane C-C shown in Fig. 2B. In Fig. 2A, a freezer 200 has an insulated casing 201, an insulated door 202 and a number of shelf members 203 for supporting goods. For the purpose of easy illustration, a coolant column 310 is shown to be attached to the back ofthe casing 201, which in practice can be built into the insulating wall ofthe casing 201, as shown in Fig. 2B. A thermal storage tank (heat tank) 400 is positioned on top of the freezer 200, but again it can be located at other places. For example, in a large system it is practical to fit the heat tank outdoor for maximum heat dissipation efficiency.

The column 310 is connected via a compressor 320 to a coolant passage 330 in the heat tank 400, which passage is connected in turn' via the tubing 340 and a solvent collector (collector) 341, to the bottom end of the column 310. The collector 341 is positioned to maintain a proper liquid level in the column 310, as to be described later. The compressor 320 is arranged to be thermally coupled with the heat tank 400, so that the heat generated during its operation is absorbed by the heat storage material in the tank 400. The control unit 500 is fitted to the top front face ofthe freezer 200.

Within the freezer 200, the cold storage tank (cold tank) 210 is fitted to the ceiling of the inner space, which tank 210 has a bottom wall 211 made of a thermally conductive material and the evaporator 150 is fitted on the bottom wall 211 so that they form an integrated cold-generating member. A brine 212 is filled in the cold tank 210, which keep the evaporator 150 submerged. An commercially available antifreeze solution can be used as the brine 212. The concentration of the solution is carefully controlled to ensure it has a freezing point a few degrees below the temperature to be maintained in the freezer. This makes it possible to use the liquid as cold storage material when the temperature in the freezer is deliberately brought to its freezing point.It is worth mentioning that once there are ice crystals formed in an antifreeze liquid, the remaining liquid part will have a higher concentration of antifreeze compound so a lower freezing point. That is to say the liquid does not have a single freezing point as pure water, instead it will freeze over a temperature range. Eventually the liquid will freeze into a thick slush which stores latent cold energy, to be released later when the slush melts. A headroom is kept in the cold tank 210 to cope with the expansion ofthe liquid 212 when it is frozen. The temperature ofthe liquid is monitored by the sensor 513 in the tank 210, while the air temperature inside the freezer 200 is monitored by the sensor 512, for control purposes to be explained later. Underneath the cold tank 210 is fitted a deep-freezing/defrosting system 600, to be described later.

As also shown in Figs. 2B and 2C, on each of the four side-walls, including the inner surface ofthe door 202, is formed a circulating path 220 which connects the cold tank 210 to a collecting chamber 221 formed on the bottom wall of the casing 201. Flexible tubes 225, one ofthen being shown in Fig. 2B, are used to connect the channel 220 formed on the inner surface of the door so that the brine circulation is not affected by door movements. A small circulating pump 230 is connected to the collecting chamber 221 for returning the collected liquid back to the tank 210 via a return pipe 240. The pump 230 and pipe 240 are embedded in the insulating material of the casing 201.Two vertical corner channels 633 are shown in Fig. 2B, which provide air circulation passages as to be described later. The casing 201 and the door 202 are made by moulding plastic materials, preferably gasifyed plastics, which provides good strength and thermal insulation.A number of internal ridges 204 are formed to enhance their mechanical strength, which also provide good attachments for the foamed insulating material The pump 230 can be of any conventional type, but it is preferable to us a small axial flow and block-free pump as disclosed in my co-pending patent application entitled "Axial Flow Pump/Marine Propeller", of an application no.4ia665l 6.,To ensure full disclosure of the inventive concept, a copy of that application is attached as reference material, and its disclosure is incorporated herein by reference.

As shown in Fig. 2C, the channel 220 is defined by channel members 223 formed on a flat panel, which are also made by moulding a plastic material, and covered by a flexible sheet 222 which is preferably a laminated sheet with at least one metal foil layer. A central support member 224 is formed in the middle of the channel to enhance the attachment of the flexible sheet to the channel base. The members 223 and 224 provide main support to the weight of the liquid in the channel 220, which reduces the stress subjected to by the flexible sheet 222.

In this way, a layer of antifreeze liquid 212 is formed by the serpentine channels 220 which cover virtually the entire inner surface of the freezer 200, and in a large system, such channels can also be formed on the shelf members 203, to flirther increase the overall size of the heat exchange surface. By using the nexible sheet 222, which is very thin and thermally conductive, a good heat exchange surface is formed between the liquid 212 in the channel 220 and the interior of the freezer 200. On the other hand, the flexibility makes the sheet 222 well adapted to cope with the expansion ofthe liquid when it is frozen in its cold storage mode, as mentioned above.Since the laminated sheet 222 has plastic cover layers, it is stable against any potential corrosive effects of the antifreeze compound to its metal foil layer(s).

Such sheets are used, e.g. for food and beverage packaging, and they can be easily attached to the supporting members 223 and 224 by adhesive or thermal welding, with or without fUrther fastening means. It is clear from the above description that except the cold tank 210, the evaporator 150 and the defrosting system 600, the whole freezer body 201 and the door 202 can be made of plastics by moulding, therefore having a better thermal insulation and also lower costs of material and manufacturing. It also makes the whole casing easily recyclable after its service life.

Figs. 3A and 3B are top views showing two embodiments of the heat tank 400 of Fig. 2k The differences between these embodiments are that Fig. 3A shows a serpentine vapour passage 330 while Fig. 3B shows a coil passage. Fig. 3C is a cross-sectional view taking along the plane C-C shown in Fig. 3k As shown in Figs 3A to 3C, the heat tank 400 has a generally flat casing formed by two casing members 401 which can be identical in structure. Each member 401 has a number of external ribs 402 and internal section walls 403 which separate the interior of the tank 400 into a number of chambers as shown in Fig 3A or a central circular chamber 401' and an outer coil chamber as shown in Fig. 3B. It is worth mentioning that in use the central chamber 401' provides a hot spot convenient for thawing frozen product taken out ofthe freezer.A coolant passage 330 is formed by two flevable and thermally conductive sheets 332, which can be of the similar type as the sheet 222 shown in Fig. 2C. The two sheets 332 are clamped between the two casing members 401 and separated by a supporting member 331 in the form of a perforated pipe or a coil of a spiral wire. The function ofthe supporting member 331 is to separate the two sheets 332 so as to prevent the passage 330 become blocked when there is a low pressure in the passage, as to be explained later. A number of internal ridges 404 are formed on the inner surface of the casing member 401, to flirther increase its mechanical strength, and also provide support to the sheets 332 when they are inflated by an internal pressure, as shown by the dash lines 332' in Fig. 3C. A phase change heat storage material 410 is filled in each chamber ofthe casing for receiving heat transferred via the sheets 332 and storing the same in the form of both sensible heat and latent heat of fusion when its temperature is raised to its melting (fusion) point.

The heat storage capacity of the tank 400 should be large enough to cope with the need of hot weather. To increase the storage capacity and the heat exchange efficiency of the tank 400, it is preferable to have different heat storage materials 410 in different chambers, so that the chamber at the vapour inlet end of the passage 330 has a material of a higher phase change temperature (fusion point) than those to the downstream end of the passage 330. In such an arrangement, a temperature gradient is formed along the passage 330 and each chamber may absorb heat from the vapour flow over a temperature range so that heat is stored evenly along the whole length of the passage 330.For control purposes, this temperature is monitored by the sensor 511 which is embedded in the heat storage materieL The preferred phase change temperature at the inlet end can be about 75"C which is below the vapour output temperature from the column 310, while at the outlet end it can be about 30"C which should be above a practical high ambient temperature, to ensure that the material will not absorb heat from ambient air during a hot summer day. These temperature values can be easily adjusted by selecting different heat storage materials to meet the needs of different climatic conditions. For this purpose, many suitable heat storage materials can be used in the tank 400, which can be hyrated inorganic salts and their eutectic mixtures or reciprocal salt pairs.The suitable examples include: calcium chloride hexahydrate (of a fusion point of 29"C); sodium sulphate decahydrate (of a fusion point of 32"C); calcium bromide tetrahydrate (of a fusion point of 33.8"C); calcium bromide hexahydrate (of a fusion point of 34.3"C); zinc nitrate hexahydrate (of a fusion point of 350C); sodium carbonate decahydrate (of a fusion point of 35"C); disodium hydrogen phosphate dodecahydrate (of a fusion point of35.50C); sodium thiosulphate pentahydrate (of a fusion point of 50"C); sodium acetate trihydrate (of a fusion point of 58 C); and barium hydroxide octahydrate (of a fusion point of 75 C).

When the heat tank 400 is at a cool status, ie. all the heat storage materials are in frozen status, a vacuum space 420 as shown in Fig. 3C is formed in the casing 401 above the top surface of the heat storage material 410 for accommodating the inflation of the passage 330 during its operation, as shown by the dash line 332'. Fluid communication arrangement (not shown) is formed between the spaces above and below the passage 330 to allow the material 410 to flow in each chamber when it melts. The interior of the passage 330 is also in low pressure under this condition so that the tank is ready to accept vapour output from the column 310.When vapour enters the passage 330, it will condense on the inner surface of the sheets 332 and give up heat to the heat storage material 410, which in turn would store it, mainly in the form of heat of fusion, and eventually dissipate it to ambient air by natural convection via the casing 401 and its inner ridges 403 and 404 and outer ribs 402 which are thermally conductive.

The casing can be made of metal materials, but to reduce cost, it also can be made of plastic materials which are moulded to form the casing member 401, then covered by a thermally conductive coating, or covered on each surface by a metal layer to increase its thermal conductivity. The checked pattern of the inner ridges and outer ribs ensure that the casing has enough mechanical strength to undergo the internal pressure changes and they also serve as fins for heat dissipation. The casing wall is made relatively thin to further improve thermal conductivity. The plastic material also has the advantage that it is stable against corrosive effects ofthe heat storage material 410. Again, the main considerations are to provide high efficiency together with low cost and recyclable structure.

Fig. 4A is a sectional view of one embodiment of the coolant column 310, with the compressor assembly 310 in it fully exposed but not in section, and Fig. 4C is a partial view of another embodiment of the column. Fig. 4B is a cross-sectional view taking along the plane B-B shown in Fig. 4C. In Fig. 4A, the coolant column has a tubular housing 310. The upper end of the housing 310 is a vapour outlet connected to the compressor 320 and its lower end is a coolant inlet connected to the collector 341 via the control valve 350. Inside the housing 310 is arranged the compressor assembly 110 with its upper end connected to a suction line 108 covered by a thermal insulation layer 109, which leads to the accumulator 160, and its lower end connected to the condenser 120 which is formed by a spiral coil of metal tubing leading to the dryer 130, as shown in Fig. 1.A flirther compressor can be used between the accumulator 160 and the pipeline 108sto pre-compress the reffigerant thus to increase its pressure and temperature before it enters the housing 310.

The compressor assembly 110 has a series of compressors 110A, 110B and 110C and chambers for compressed reffigerant, 11 it, 11 1B and 11 lC, forming a multistage chain through which the reffigerant is compressed progressively. The compressors 110A to 110C are preferably a free-piston type as disclosed in my co-pending patent application entitled '!Reciprocating Motor and Compressor Incorporating the Same", of application no.(#hf+ To ensure a full disclosure of the inventive concept, a copy of that application is also attached herewith as reference materiaL Its disclosure is incorporated herein by reference.

More particularly, the tubular compressor 110A is connected to the chamber 111A, which in turn is connected to the next stage compressor 110B, and so on. The outer surface of the series of three compressors and three chambers can be formed by a single cylindrical member made of thermally conductive material, so as to achieve good heat exchange.

Internal fins can be formed inside each of the chambers 11 lA to 11 1C, as shown in Fig. 4B, to further enhance heat exchange efficiency, and also mechanical strength so that the side wall of the cylindrical member can be made relatively thin. The number of compressors and chambers can be changed to meet the needs of different reffigerants or application requirements. That is to say, the whole arrangement is made highly flexible and adaptable to meet different needs.

From the upper end of the assembly 110 downwards are fitted in sequence a cup member 115, a collar member 114, an inner heat exchange member 117 surrounding the chamber 111A, and a spiral fin 116 extending from the compressor llOB down to the last chamber 11 1C, which in turn is connected to the condenser 120. A sensor 514 is fitted close to the top end of the fin 116. As also shown in Fig. 4B, the four thermoelectric members 112 are circumferentially fitted between the outer annular surface of the inner heat exchange member 117, which has fins extending inwards to form thermal contact with the chamber 111A, and the inner annular surface of an outer heat exchange member 113 which has radially outward extending fins. Their functions are to be explained later.Within the tubular housing 310, it is fitted close to its upper end a vapour separator 311 which is a porous or perforated board allowing vapours to pass through but not any liquid droplets carried up by a vapour flow. Below the heat exchange member 113, there is a flow guide member 312 which has a tubular portion 316, a collar portion 313 at the upper end of the tubular portion 316, and a number of parallelly arranged spiral flanges 315 formed on the outer surface of the tubular portion 316. At the bottom end of the housing 310 is fitted a nozzle 352 which has an internal one-way valve 351 allowing a coolant outflow from the nozzle but not a flow back into it.A particle separator 353 is fitted to cover the no:::zle 352, which separator functions as a filter allowing liquid from the nozzle 352 to pass through but not allowing solid particles to settle into the nozzle.

The coolant solution is preferably formed by a cocktail of aqueous ammonia with one or more endothermic salts dissolvable in it. Suitable salts include ammonium nitrate, potassium thiocyanate, ammonium chloride, potassium nitrate, urea etc. or a mixture of any of theia The concentration of the aqueous ammonia is made below saturation within the normal range of ambient temperature, ie. in the concentration range of 20-35 wt.%, its exact value is to be determined according to local climate. A salt mixture of ammonium nitrate and urea is preferable for their low commercial cost (both are used as common chemical fertliser). Carbon dioxide can also be included in this cocktail to increase its evaporable contents.

From the operational point of view, the internal space of the column housing 310 is divided into the following functional areas. Firstly, the top portion forms a vapour chamber 301 in which ammonia and water vapours rise upwards to the vapour outlet. Below the vapour chamber 301 is a boiling zone 302 which is formed by the thermoelectric members 112 and its outer heat exchange member 113. The space below the boiling zone 302 is divided by the flow guide member 312 into an outer precipitation zone 303 which is the annular space between the housing 310 and the outer surface of the tubular portion 316 of the flow guide member 312, and an inner evaporation zone 304 which is the annular space between the inner surface ofthe tubular portion 316 and the outer surface ofthe compressor assembly 110, and the zone extends upwards to the cap 115.Below the flow guide member 312 is a salt chamber 305 which is connected at its bottom to the coolant inlet nozzle 352.

The operation of the coolant circuit 300 is now described with reference to Fig. 3A to 3C, 4A and 4B. When the compressor assembly 110 starts working, it sucks in gaseous refrigerant from the suction line 108 and compresses it progressively through the chain of the compressors 110A to 110C and the chambers 111A to 111C. During this process the temperature of the assembly 110 and condenser 120 begins to rise because of the pressure increase. At the same time, the electromagnetic valve 350 is opened to allow the aqueous ammonia in the collector 341 to enter the salt chamber 305 through the nozzle 352 and the salt separator 353 and to form a upward counterflaw to reach a liquid level in the lower part of the boiling zone 302.The liquid level is stabilised within the zone 302 due to the relative vertical position ofthe collector 341 which supplies the solvent.

During this process a quantity of salt particles in the chamber 305 is dissolved by the aqueous ammonia solvent, producing endothermic effects to reduce the temperature in the salt chamber 305 to about 0 C, therefore the part of the condenser coil 120 within the chamber 305 is cooled and the reffigerant in it is properly condensed to liquid although the total length of the coil is very short. The saturated salt solution formed in the salt chamber 305 flows upwards into the inner evaporation zone 304 defined by the inner tubular surface ofthe flow guide member 312. Once this upward flow comes into contact with the upstream part of the condenser 120 and the compressor assembly 110, which are at a higher temperature as explained above, the temperature of this saturated solution begins to rise.

This has the effects that on the one hand it reduces the solubility of the ammonia in the solution, which causes ammonia evaporation, as indicated by the small circles shown in the inner evaporation zone 304, while on the other hand, it increases the solubility ofthe salts so the solution becomes less saturated and the salts remain dissolved although its relative concentration becomes higher after the evaporation of the ammonia. Due to the existence of the spiral heat exchange fin 116, the upward flow of the coolant solution in the inner evaporation zone 304 has a very good thermal contact with the compressor assembly 110, therefore keeps it cooled. The heat transferred from the assembly 110 to the coolant flow in the zone 304 causes a temperature increase along the flow path, hence more evaporation of the ammonia, forming a strong bubbling flow.Finally this bubbling flow passes the gaps between the inner heat exchange member 117 and the chamber 111A, and is turned by the cap 115 downwards into the boiling zone 302. The temperature of this bubbling flow is constantly monitored by the sensor 514, so that the control unit 500 in Fig. 1 can adjust the flow rate via the control valve 350, and also the current through the thermoelectric member 112 to keep the working temperature for the assembly 110 stable, so as to compensate the changes of its load.

The vapours in the bubbling flow rise immediately into the vapour chamber 301, as shown by the small circles therein, while the liquid part of the bubbling flow coming out of the cap member 115 enters the boiling zone 302. The outer heat exchange member 113 in the boiling zone 302 is thermally engaged with the hot side of the thermoelectric members 112 to receive heat transferred by their Peltier's effect from the wall of the chamber 111A via the inner member 117, as shown in Fig. 4B. This arrangement is made to maintain the outer member 113 thermally elevated to the boiling temperature of the coolant solution, which is also the highest temperature in the whole column 310, so a significant amount of the water in the boiling zone is evaporated. Because of the high temperature in the zone 302, the solubility ofthe salt components in the solution is further increased to form a solution of very high salt concentration after the water evaporation. This strong solution is heavier than the solution below it in the outer precipitation zone 303, which causes it to sink through the throughholes 314 formed in the collar portion 313 of the flow guide member 312. After the strong solution enters the zone 303, it is guided by the flanges 315 to flow downwards and at the same time losing its heat to the solution in the inner zone 304 through the thin wall 316.

As this solution is cooled progressively during the downward flow, the solubility of its salt contents decrease so the salts begin to form crystal particles, as represented by the small cross signs in the zone 303. The salt particles would first settle onto slope surfaces of the flanges 315 then slip down into the salt chamber 305, to be re-dissolved by the incoming aqueous ammonia flow for a new cycle ofthe salt circulation.

It is clear from the above description that under the control of the unit 500 based on the value sensed by the sensor 514, a dynamic circulation is carried on between the inner evaporation zone 304, the boiling zone 302, the outer precipitation zone 303 and the salt chamber 305. A temperature gradient along the axial direction of the column 310 is therefore stabilised between the salt chamber 305, which is about 0 C, and the boiling zone 302 which is about 100"C, due to the baffling effects ofthe spiral fin 116 in the zone 304 and also the spiral flanges 315 in the zone 303, which prevent turbulent mixing ofthe liquid in each of the zones.The flow guide member 312 is made by moulding a plastic material to form the flanges 315, which are relative thick compared with the tubular wall 316, which can be made of thermally conductive materials, e.g. by moulding the flanges 315 around a metal tube 316, so that the heat exchange in the radical direction between the inner and outer zones 304 and 303 is promoted against the heat exchange in the axial direction within each zone.

The column housing 310 is made of plastics and has a heat reflective inner surface, so it provides good thermal insulation which helps to maintain the temperature gradient in its axial direction. The system efficiency is therefore significantly improved due to two mutually supporting factors which reduce energy losses during its operation. Firstly, the whole compressing chain is cooled by the coolant flow which reduces the compressor loss and also takes a significant amount of heat away from the reffigerant before it enters the condenser 120. Secondly, the cooling effects in the salt chamber lower the condensation pressure of the refiigerant in the condenser, hence reduce condenser loss and throttling loss in the system.The reduced condensation pressure makes it practical to use environmentally benign refrigerant, such as carbon dioxide or anhydrous ammonia which would otherwise condense only under a much higher pressure.

At the top end of the column 310, a mbuure of ammonia and water vapours in the vapour chamber 301 form an upward flow promoted by the sucking force of the compressor 320, which can be of the same type as the compressors in the assembly 110. Between the compressor 320 and the thermoelectric members 112, a mutually compensating relationship is formed to encourage the water evaporation by maintaining a low pressure in the chamber 301, which lowers the boiling temperature of the salt solution in the boiling zone 302 to improve the efficiency ofthe members 112, or vice versa.

Refer to Fig. 3C, when the vapour mixture enters the passage 330, its water content would be the first to condense onto the flexible sheets 332 and give up its heat to the heat storage material 410. Then the condensed water would absorb the ammonia vapour to form an aqueous solution. This process slows down pressure build-up in the passage 330 to make room for vapours coming afterwards. By using the flexible sheets 332, the passage 330 can be inflated under the vapour pressure to increase the total size of the heat exchange surface hence the rate of water condensation and ammonia absorption. The ammonia absorption is an exothermic process and the heat so evolved is transferred to the heat storage material 410.

Finally, the aqueous ammonia formed in the passage 330 would flow via the connection tubing 340 into the collector 341 for another cycle ofthe solvent circulation. To encourage the return flow, the heat tank 400 is tilted towards the outlet end of the passage 330 so the flow is achieved manly by gravity, however, the actual flow rate is decided by the valve 350 under the control ofthe unit 500.

When the compressor assembly 110 stops operating when the temperature inside the freezer is below a predetermined value, and the valve 350 is shut to stop the incoming flow of the aqueous ammonia solvent. At the same time, the compressor 320 is switched to a low-power operation which allows out-flow of the vapour mixture, so that the coolant solution in the column 310 cools down gradually by further evaporation which causes more salt to precipitate and sink to the salt chamber 305. Similarly, the heat tank 400 would also cool down by dissipating stored heat to ambient air, therefore causing the vapour inside the passage 330 to condense and dissolve thus to create a low pressure in the passage 330.Due to the continuing operation ofthe compressor 320, which does not consume much power but acts more like an active one-way valve, low pressure is also established in the vapour chamber 301, which in turn causes more water evaporation, therefore cools the interior of the column further, although the column housing 310 itself provides a good thermal insulation which prevents heat dissipation through its walL The one-way valve 351 and the control valve 350 help to prevent any salt from entering the pipeline 340. At this final status, most of the salt content is accumulated as solid particles in the chamber 305 while most of the solvent, ie. aqueous ammonia, is accumulated in the passage 330 and the collector 341.

Because the temperature in the tank 400 and the collector 341 will eventually approach ambient (e.g. room) temperature, the aqueous ammonia collected in them would be below saturated concentration, ie. forming a weak solution ready to absorb ammonia vapour in next session. This is highly beneficial when the compressor assembly 110 works intermittently with relatively short operating periods and long idle periods so that ammonia is evaporated mainly during the operating periods while water over all periods.

Fig. 4C shows a second embodiment ofthe coolant column 310. The differences between this embodiment and that of Fig. 4A are that the cap 115 in Fig. 4A is replaced by a valve assembly formed by a valve member 115', a support member 118 and a spring 119, and a flap valve 319 is attached beneath the collar portion 313 of the flow guide 312 to cover the through-holes 314. In operation, the valve member 115' is biased by the spring 119 to keep the inner evaporation zone 304 closed, at the same time the flap valve 319 closes all the through-holes 314. Alternatively, the member 115' can be biased by magnetic force, with or without the spring 119. That is to say, the boiling zone 302 and the vapour chamber 301 are separated from the inner and outer zones 304 and 303, to allow a low pressure to be formed by the effect of the compressor 320.The low pressure and the elevated temperature of the heat exchange member 113 in the boiling zone work together to drive water to evaporate so that a proper proportion of water vapour will enter the tank 400 with the ammonia vapour to form the aqueous ammonia. During this time, a high pressure is built up in the inner evaporation zone 304, which would, in combination with the low pressure in the vapour chamber 301, eventually overcome the biasing force on the valve member 115' and force it to open. Once this happens, a bubbling flow will rush into the boiling zone, and at the same time the flap valve 319 will also be opened to allow the high concentration solution in the boiling zone to enter the outer precipitation zone 303 below. In this way an intermittent local circulation of the coolant solution is formed which balances the amounts of water and ammonia vapours formed in the process. For control purposes, the average temperature value provided by the sensor 514 is used by the unit 500 to balance this intermittent operation. Due to this balanced evaporation of different vapour components, the embodiment can be used for continuous operation or for intermittent operation with relatively short idle periods.

Also shown in Fig. 4C is a second thermoelectric arrangement at the lower end of the assembly 110, which is formed by the inner heat exchange member 117', thermoelectric members 112' and an outer heat exchange member 113'. They work in the same way as the members 112, 113 and 117 described above, but in an opposite direction, ie. the member 113' forms the cold side ofthe arrangement so it cools the solution in the outer zone 303 to encourage more salt precipitation. Again it is used to maintain the temperature gradient in the column 310, as described above.

Fig. 5A is a sectional view illustrating details of the defrosting system 600 in Fig. 2A, and Fig. 5B is a sectional view taking along the plane B-B in Fig. 5k In Figs. 5A and 5B, the defrosting system 600 has a thermoelectric member 610 attached to the bottom wall of the cold tank 210, and a heat exchange member 620 fitted beneath it. The member 610 is formed by an array of Peltier elements which are surrounded by an insulating member 611 for separating the heat exchange member 620 from the cold tank 210 and also for absorbing thermal stress between them A space 630 is formed below the tank 210 by a hinged member 631 which also serves as an ice collecting member, as to be desonDed later, and a separation board 632.Two corner channels 633, also shown in Fig 2B, extend from the space downwards to the bottom of the freezer 200. Two small fins 640 are each fitted to one back corner of the space 630 to draw in an air flow from the front edge of the heat exchange member 620, then to drive the air out via each of the corner channels 633 into the inner space ofthe freezer.

It can be seen from the sectional view of Fig. SB that the heat exchange member 620 has a base 621 which is in direct contact with the bottom surface of the thermoelectric elements 610, and a number of fins 622 extending from the base downwards, separating the space between the base 621 and the hinged member 631 into a number of parallel channels.

A number of concave cells 623 are formed on the base, each corresponding to one of the thermoelectric elements, their function is to be explained later. An ice detecting system is formed by a light emitting diode (LED) 661 fitted to one side ofthe heat exchange member 620 with a light sensor 662 fitted opposite to it. A series of small holes 663 are formed in the fins 622 allowing a light beam 664 from the LED 620 to pass through and be received by the sensor 662.

The defrosting arrangement 600 can work in two different modes, ie. the first mode for deep-freezing and the second mode for defrosting/ice collecting.

When the arrangement 600 operates in the first mode, an electric current is supplied to the thermoelectric element array 610 in a first direction, in which the top surface of each element 610 warms up and gives heat to the evaporator 150, while the bottom surface of each element becomes colder, cooling the member 620 to a temperature lower than that of the evaporator 150 and the antifreeze liquid in the cold tank 210. That is to say, the heat exchange member 620 becomes the coldest part inside the freezer 200, which can be well below -30 C, ie. lower than the operation range when ammonia is used as refrigerant. At the same time, the fans 640 cause an air flow passing through the channels defined by the fins 622, in which the air is dehydrated and its moisture content becomes frost.Then the dry and cold air is driven by the fans 640 down into the corner channels 633 to be released at the bottom into the inner space of the freezer 200. Because the temperature in the concave cells 623 is lower than at any other position in the freezer, frost accumulates in these cells, which helps to keep other parts of the freezer frost-free. When a certain amount of frost/ice is collected in the cells 623, the light passage 663 will be blocked and this will be sensed by the sensor 662, so the control unit 500 can switch the operation to the defrosting mode.

In the defrosting mode, a large current of reversed direction is supplied to the thermoelectric elements 610, which has the effect of causing them to reverse their operation to absorb heat from the cold storage tank 210 and transferring the same to the heat exchange member 620 to cause a sharp raise of its temperature. This temperature change of the member 620 causes a thermal expansion of the side walls of the concave ice cells 623 which produces a thermal shock to break the accumulated ice, which is brittle. The broken pieces of the ice then drop onto the ice collecting member 631, forcing it to swing downwards, as shown by the dash line 631', to deliver the ice pieces into an ice collecting pocket 650 attached to the inner surface of the door 202.After the frosthlce accumulated on the member 620 has been cleared, the sensor 662 would be able to receive light signals from the LED 661, and the operation can be switched back to the first mode.

It is advantageous that during the defrosting operation, the frost/ice accumulation is physically expelled by the thermal shock produced by the heat exchange member 620, and to achieve this effect the amount of energy and the length of time needed are very small because it does not need to melt the ice, as in case of most conventional defrosting arrangements. On the other hand, the ice is formed by pure water, so the pieces themselves are valuable commodities ready for human consumption. In this sense, nothing is wasted during the operation ofthe arrangement. Obviously, this arrangement can also be used for the purposes of ice-making or quick-freezing.To achieve this, a control button can be fitted to the control unit 500 so that a user can make ice or quickly freeze fresh goods by putting a tray of water or the goods into the freezer 200, suitably at the bottom close to the air outlet of the corner channels 633, then starting the deep-freezing mode of the arrangement 600 by pressing the control button. In due course, the defrosting mode would be actuated then an indicating light signals that the ice pieces are collected in the pocket 650, ready for use.

A method for controlling the operation of the cooling system according to the present invention is described hereinbelow with reference to Fig. 1 and Fig. 6.

The operation of the control unit 500 in Fig. 1 is based on constant evaluation of four temperature values including: tl, sensed by the sensor 511 fitted in the heat storage tank 400, indicating the real time temperature of the heat storage material at the outlet end of the passage 330; t2, sensed by the sensor 512 fitted in the freezer 200, indicating the internal temperature ofthe freezer; t3, sensed by the sensor 513 fitted in the cold storage tank 210, indicating the temperature of the antifreeze liquid in the tank 210; and t4, sensed by the sensor 514 fitted in the inner evaporation zone, as described before.In practice, especially in a large system, each of the sensors 511 to 514 can be formed by a set of sensing elements fitted at different positions of the relevant components, and the relevant values of tl to t4 would be the average values of each set of the sensing elements. The temperature sensor 511 in the heat storage tank can also be replaced by a pressure sensor for the same control purpose, then the control can be conducted on the basis of the vapour pressure in the passage 330.To facilitate the description, predetermined reference values of a set of control parameters for each parts of the system are marked in Fig. 1, which are suitable, e.g. for a standard four star (* * * *) freezer, in which the temperature inside the freezer, ie. t2, should be controlled below -14"C. Corresponding to this value, the freezing point of the antifreeze liquid in the cold storage tank 210 is adjusted to about -20 C, that is to say ice crystals begin to appear in the liquid at -20 C, and it can be considered as fully frozen when below -28 C. On the other hand, for practical purposes the highest expected room temperature, e.g. in England, is assumed to be below 30"C so this value is selected as the required fusion point ofthe heat storage material at the outlet ofthe passage 330 in the heat tank 400. In operation, as long as tl is not beyond this fusion point, one can assume that the tank 400 has not been charged to its full heat storage capacity. Obviously, these values are given as examples, and they should be adjusted when conditions change.

From control point of view, the system can operate in any ofthe following modes: (a) Full capacity cooling operation (Full cooling) In this mode, the refrigerant circulating circuit 100, the coolant circulating circuit 300, the brine circulating circuit 200 and the thermoelectric system 600 (in its deep freezing mode) are all operating at their respective full capacity. This is achieved by the control unit 500 by actuating the compressor assembly 110 and the thermoelectric members 112, the compressor 320, the control valve 350, the circulating pump 230, the thermoelectric member 610 and the fans 640 (shown in Fig. 5A).

(b) Deep freezing operation (Deep freezing) In this mode, it is intended to achieve the full capacity cold storage in the cold storage tank 210. The system operation is basically the same as that of mode (a) except that the circulating pump 230 is turned off to allow the antifreeze liquid to freeze.

(c) Economic cooling operation (Economic cooling) The difference between this mode and the above mode (a) is that the thermoelectric system 600 (including the fans 640) does not work so the power consumption is lower.

(d) Air circulating operation (Air circulating) In this mode, only the fans 640 work to circulate air within the freezer 200. The air is cooled in the space 630 by the frozen antifreeze liquid in the cold storage tank 210.

(e) Defrosting operation (Defrosting) In this mode, the system 600 is switched to its defrosting mode as described before.

It will switch back automatically when the frost/ice accumulation has been cleared, as mentioned before. This operation can happen during the full capacity cooling operation mode (a) or the deep freezing operation mode (b).

(f) Pause In this mode, the control unit 500 turns the system off for a predetermined period, e.g. of 10 minutes. All the operational components are off except the relevant sensors for monitoring condition changes, but the compressor 320 is kept operating at a low power status as an active one-way valve for at least a part of this period, as explained before. This mode is prolonged when the freezer door is open and terminated when the door shuts.

Fig. 6 shows the control logic of the control unit 500, which is based on a CPU with necessary supporting components. In Fig. 6, the first step S000 is the starting point of a control session and also the point that the program returns i.e. form step S006 after each session. Once the process is started, the first thing is to check at step S001 whether the freezer door is open. If the answer is 'Yes'= the operation is switched to the mode "Pause" at step Soys. As mentioned above, this mode is maintained as long as the door is kept open and terminated when the door is shut. Alarming arrangement can be incorporated into the unit 500 to alarm a user when the door is kept open for too long or not being shut properly.

If or once the door is shut, the program goes to the next step S002 to read from the sensors 511 to 514 the respective values tl, t2, t3 and t4. Then in step S003,thevalueoftl is checked to see whether the heat tank 400 is charged to its full capacity. If the answer is "Yes", it goes to the step S005 to pause for a while, allowing the tank to cool down.

Otherwise it goes to the step S004 to see whether the low cost electricity is available. This can be done by checking the voltage of different power input terminals. In case of using offpeak electricity, this step is simply to check a timer in the control unit to see whether it is the right time. If the answer is "Yes", the system is switched to the low-cost electricity then executes step S100, in which it checks whether t3 is below -28 C to see whether the cold tank 210 has been charged to its full capacity. If the answer is "Yes", it goes to step S005 to pause e.g. for ten minutes, then returns via step S006 to the starting point S000.If the answer at step S100 is 'No", ie. the tank 210 is not fully charged, the control unit executes step S101 to check the value of t3 to see whether the antifreeze liquid in the tank 210 is partially frozen. If the answer is "Yes", the system starts the "Deep freezing" mode at step S103, otherwise it goes to step S102 to see whether t3 is below -14"C. If it is "Yes", it operate in "Full cooling" mode at step S104 when the antifreeze liquid can circulate. Atter the operation is started, the ice detecting system checks at step S105 to actuate the defrosting operation at step S106 if the answer is "Yes", then it returns.If the answer at step S102 is 'No", it indicates that the antifreeze liquid is very warm, it goes to step S207 for "Economic cooling". This would avoid the system to be overloaded, e.g. during its start-up.

Then it returns for next session of the control process.

If at the step S004, it is found that the low-cost electricity is not available, the control unit 500 executes step S200 to see whether the value of t2 is above 0 C, ie. whether the interior of the freezer 200 is unacceptably warm. This may occur when the freezer is started, its door is kept open for a long time or a large quantity of fresh goods is loaded into it. If the answer is "Yes", it goes to step S102, then the following steps would be the same as that mentioned above. If the answer at step S200 is 'No", it checks at step S201 to see whether the ice-making/quick-freezing button is pressed.If the answer is 'Yes", it goes to step S101 to decide either to start full cooling or deep freezing operation according to the value of t3.

If the answer at step S201 is "No'= ie. no instruction for deep-freezing, the value of t2 is checked at step S204 to see whether the internal temperature is acceptable, ie. below -14"C.

If the answer is ';Yes", the system is switched to '?ause" mode at step S005, then restarts, otherwise the value of t3 is checked at step S205 to see whether the antifreeze liquid is still frozen. If it is 'Yes", the system starts the air circulation by the fans 640 at step S206, during which the interior of the freezer is cooled by the cold energy stored in the cold tank 210. In practice, this operation would be able to cope with a temporary change of the internal temperature caused, e.g. by the invasion of warm air when the freezer door is opened briefly. If it is "No" at step S205, the system operates at step S207 in "Economic cooling" mode which is the mode when the low-cost electricity is not available.

It should be noted that once one ofthe operation modes in step S103, S104, S206 or S207 is started, the control unit 500 does not stop the operation until it finds at a later session that one of the control criteria for stopping the operation exists, ie. at step S001, S003, S100, or S204.

It is to be understood that by selectively switching between different modes, the system as a whole achieves a significantly improved efficiency. For example, if the system is designed to make full use of off-peak electricity at night, by switching the system to full cooling mode at step S104 or deep freezing mode at step S103, the interior ofthe freezer is cooled to below -28 C. In this case, a very large quantity of cold energy is stored not only in the frozen antifreeze liquid in the cold tank 210 and the channels 220 shown in Figs 2A to 2C, but also in the goods supported on the shelf members 203.

The stored cold energy would be able to keep the cooling mechanism idle for a long period after the low cost electricity is no longer available, therefore reducing the amount of "expensive" electricity used during daytime. In this case, since the cold energy is stored during the period from midnight to early morning, when the ambient temperature is the lowest and also it is lest possible for the freezer door to be opened, the overall cooling efficiency is highest.

By making full use of the off-peak electricity which leads to reduced use of peak-time electricity, it also helps to alleviate imbalance of demands to a national grid, therefore contnbutes to the improvement of nation-wide power efficiency.

Industrial Applicability In the above description, embodiments of a freezer are used to illustrate operating principles ofthe inventive concept. It would not be difficult for those skilled in the art to see that the concept can be used in other applications which require efficient cooling, for example, it can be easily adapted into a ffidge with or without a freezer compartment, a refrigerated display case or cabinet, a cold room, a refrigerated vehicle, vessel or aeroplane, or the cooling portion of an air-conditioning system.

The control concept regarding the utilisation of the off-peak electricity can be equally applied to other kinds of low-cost electricity, such as solar, wind or tidal power, or in case of a cooling system for a mobile application, the ground power in contrast with the electricity supplied by an on-board generator. Obviously different kinds of low-cost electricity can be used in combination, e.g. using solar power during the day and the off-peak power from national grid at night, combined with wind power whenever it is available. Furthermore, diffi rent components, parts or special functions of the system as disclosed above can be adjusted, deleted or rearranged by those skilled in the art to meet particular requirements of different applications without departing from the concept of the present invention. For example, other evaporable component(s), such as methanoL ethanol and/or carbon dioxide can be used in the coolant solution in case ammonia is not suitable, e.g. when a system has copper parts; the heat storage tank may not be necessary for a system in which heat dissipating is not a problem; and the ice-rnakiag/quick-freezing facility would not be needed for air-conditioning or high temperature refrigeration. In the latter case, a cold storage gel of a higher freezing temperature, which is known in the art, can be used to replace the antifreeze liquid for storing cold energy at e.g. about 0 C, suitable for air-conditioning or high temperature refrigeration. Since all these elements are well-known in the art, they do not need further explanation.

Claims

1. A cooling system comprising: a primary cooling mechanism for transferring heat from a cold-generating member to a heat-rejecting member, and a coolant circuit thermally engaged with said heat-rejecting member of said mechanism; wherein a coolant solution with at least one evaporable component is used to circulate in said coolant circuit which has means for promoting evaporation of said evaporable component to improve energy efficiency of said primary cooling mechanism by dissipating heat from said heat-rejecting member by said evaporation.

2. A cooling system of claim 1, wherein said coolant circuit includes at least one endothermic salt for cooling said heat-rejecting member further by endothermic dissolution of said salt in said solution.

3. A cooling system of claim 1 or claim 2, wherein said coolant solution has at least two evaporable components, each of a different evaporation temperature.

4. A cooling system of claim 3, wherein said coolant solution includes aqueous ammonia.

5. A cooling system of claim 3 or claim 4, wherein said coolant solution includes carbonated water.

6. A cooling system of any one of the preceding claims, wherein said evaporation promoting means comprises arrangement for maintaining a low pressure in a portion of said coolant circuit.

7. A cooling system of any one of the preceding claims, wherein said evaporation promoting means further comprises a thermoelectric member arranged with its cold-side thermally engaging said heat-rejecting member and its hot-side engaging said coolant solution so as to provide thereto an elevated temperature for increasing said evaporation.

8. A cooling system of any one of the preceding claims, wherein said evaporation promoting means further comprises a vapour pump for creating a low pressure portion in said coolant circuit.

9. A cooling system of any one of the preceding claims, wherein said coolant circuit further comprises: a heat absorbing portion thermally coupled with said heat-rejecting member; and a heat dissipating portion in fluid communication with said heat absorbing portion for dissipating heat away from said heat-rejecting member; wherein said heat absorbing portion is arranged to have an upper part with a coolant outlet, an lower part with a coolant inlet, and an intermediate zone engaging said heatrejecting member, said intermediate zone has baffle means for stabilising a temperature gradient between said upper and lower parts so that said evaporable component of said solution can evaporate in said upper part and flow to and condense in said heat dissipating portion and then return to said intermediate zone via said lower part.

10. A cooling system of claim 9 in combination with claim 2, wherein said heat absorbing portion further comprising a flow guide means which separate said intermediate zone into a first part defining an evaporation zone in thermal contact with said heat-rejecting member in which said component of the coolant solution evaporate, and a second part defining a precipitation zone separated from said heat-rejecting member but in fluid communication with said first part so that said endothermic salt of the coolant solution can precipitate and settle to said lower part.

11. A cooling system of claim 10, wherein a salt chamber is formed at said lower part of the heat absorbing portion to have said endothermic cooling effect to said heat-rejecting member.

12. A cooling system of any one of claims 9 to 11, further comprising a coolant collector connected between the coolant inlet of said heat absorbing portion and the outlet end of said heat dissipating portion; wherein said collector is positioned to maintain a predetermined liquid level in said heat absorbing portion.

13. A cooling system of any one of the preceding claims, further comprising: a brine circuit thermally engaged with said cold-generating member for improving its heat-exchange efficiency.

14. A cooling system of any one of the preceding claims, further comprising a heat but arrangement thermally engaged with said coolant circuit for temporarily storing and for dissipating heat transferred by said coolant solution.

15. A gas compression assembly for a cooling system of any one of the preceding claims, comprising: a plurality of compressors serially connected to form a multistage gas compression chain in which a refrigerant is to be progressively compressed and then supplied to a refrigeration circuit; wherein said gas compression chain is to be arranged as said heat-rejecting member having thermal engagement with said coolant circuit so that the heat generated therein by progressively compressing the refrigerant is to be absorbed and carried away by said coolant solution circulating in said coolant circuit.

16. A gas compression assembly of claim 15, further comprising condenser means connected to the downstream end of said gas compression chain, and being thermally coupled with said coolant circuit.

17. A gas compression assembly of claim 15 or claim 16, wherein said coolant circuit comprises a column member which encloses said gas compression chain so as to have said compressors and/or condenser means submerged in said coolant solution.

18. A heat storage tank for a cooling system comprising a thermally conductive casing, a fluid passage located in said casing and being defined by a thermally conductive and flexible wall member, and at least one heat storage material filled between said casing and said fluid passage for absorbing heat from a heated fluid passing through said passage.

19. A heat storage tank of claim 18 wherein said casing is divided into a plurality of chambers having different phase-change heat storage materials, and wherein the chamber containing an up-stream portion of said fluid passage has a phase-change material of a fusion point higher than that of the material in the chamber containing a down-stream portion of said fluid passage.

20. A defrosting arrangement for a cooling system, comprising a thermoelectric member with one thermal pole coupled to a cold-generating member of said cooling system and the other thermal pole coupled to a heat exchange member, and a control unit for selectively changing electric supply to said thermoelectric member so as to reverse the heat transfer direction of the thermoelectric member between said cold-generating member and said heat exchange member, therefore selectively changing the operation status of said heat exchange member from a frost accumulating mode to a defrosting mode.

21. A defrosting arrangement of claim 20, further comprising an ice detecting arrangement for sensing the amount of ice accumulated onto said heat exchange member and indicating the same to said control unit.

22. A defrosting arrangement of claim 20 or claim 21, further comprising an arrangement for collecting ice expelled from said heat exchange member during its defrosting operation, said arrangement includes a hinged mechanism for delivering ice to a collecting pocket.

23. A method of operating a cooling system incorporating an antifreeze circuit, comprising steps of a). setting a mode control unit for selecting one of two operational modes in response to the availability of low cost electricity; b). operating the cooling system in a first mode when said low cost electricity is not available, in which said antifreeze is cooled to a temperature under which it can be circulated to improve system heat exchange efficiency; and c). operating the cooling system in a second mode when said low cost electricity is available, in which said antifreeze is cooled to a temperature low enough for it to be frozen so as to store cold energy therein.

24. A method of claim 23, further comprising a step of d). causing an air circulation within a cooled space during the operation of said second mode.

25. A method of claim 23 or claim 24, further comprising steps of: e). causing frost accumulation on a heat exchange member associated with a thermoelectric member; and f). causing defrosting by reversing the operation of said thermoelectric member.

26. A cooling system constructed substantially as descnbed herein with reference to the accompanying drawings.

27. A method for operating a cooling system, performed substantially as described herein with reference to the accompanying drawings.